One fan theory potentially makes his motivations all the more clear if it turns out to be what will eventually happens in the series. A theory postulating whether or not Jiren is actually trying to defeat Goku and Vegeta, but instead train them for a bigger battle to come.
Reddit user jl_theprofessor has a theory that Jiren hasn't outright defeated the two warriors, and his holding back his power because, "this whole tournament is a chance for Jiren to act as a teacher to Vegeta and Goku and prepare them for even greater challenges."
Citing Episode 123's battle as evidence, "when [Goku and Vegeta] both power up, [Jiren] tells them to come at him like a knowledgeable instructor who's waiting to spar with his skilled but unrefined students. And what does he say when he blocks their simultaneous hits that (admittedly) knock him back into a wall? He says, 'Both of your strikes are sharper."
According to this theory, Jiren is "like a teacher telling his students that they've improved, he has no problem letting them know they've gotten better. Of course he's aware, that was his whole purpose in letting them stick around in the first place"
The rest of this theory alludes to an even great battle on the horizon, one with the gods, and while that could still happen eventually in another incarnation of Dragon Ball, Super is going to end after the Universal Survival arc is complete.
So if this Jiren training theory pans out, fans will have to wait until the next entry in the franchise to see if Toei actually capitalizes on it.
Dragon Ball Super's "Universal Survival" arc is part of the recent simulcast agreement that sites like Crunchyroll and Funimation have scored. DragonBall Super airs on Crunchyroll Saturday evenings at 7:15 p.m. CST. Adult Swim airs the English dub during its Toonami block Saturday evenings at 9:30 p.m, and is now available to stream on FunimationNOW and Amazon Video.
There is also currently a new Dragon Ball film in the works for 2018. The film will focus on the Saiyans, the "origins of Goku's power," and potentially the story of the very first Super Saiyan God. Not only does it aim to be the best film in the series, original creator Akira Toriyama will be contributing to the film's script and new character designs.